1. Field of the Invention
The general field of the invention is artificial lift, in particular, plunger lift systems to move liquids upward in a subterranean well.
2. Description of the Related Art
Subterranean wells typically produce both liquids and gasses. Gas wells will therefore produce liquids, such as fresh water, salt water, or condensate, with the gas flow. A problem arises when liquids accumulate in the well bore of a gas well. The liquids in the wellbore will hold back and eventually stop the flow of gas to the wellbore and therefore stop production of gas to the surface. This is known as liquid loading. Liquid loading in the wellbore is often a serious problem especially in mature gas wells.
Chemical soaping, velocity string, and plunger lift are some of the techniques currently used to overcome liquid loading. Soap sticks can be dropped into the well. They produce foaming and thus reduce the fluid column hydrostatic pressure to keep the well producing. This is known as chemical soaping. As the well rate declines, slim tubing, such as a 2⅛″ tubing can be used to replace the regular tubing so that gas velocity can be increased to effectively carry liquids out of the well. This is also known as velocity string. However when the reservoir pressure declines further, chemical soaping and velocity string may no longer work. Plunger lifting has proven to be a cost effective way to improve such wells' productivity.
Plunger lift is currently used as a method of removing liquids from the wellbore using the formation gas as the energy source. Therefore a substantial amount of formation gas pressure is required to create enough force to push the plunger from the bottom of the well to the surface. The required pressure is often achieved by shutting in the well, sometimes for significant periods of time, while the gas accumulates in the annuals. This results in loss of production. In addition, the procedure may need to be performed multiple times a day, which means that the well would have to be shut on and off multiple times in a 24 hour period, further reducing the hours in a day in which the well can be producing.
If production packers are used to prevent hydrocarbons in the tubing casing annulus, for example as a safety measure, the gas would not be able to accumulate in the annulus above the packers. Because of the relatively limited space available in the annulus near the bottom of the bore hole, the volume of accumulated gas may not be sufficient to achieve the needed pressure. In addition, for mature wells with declining production rates, the drop in pressure of the well fluids will reach a point where it's not possible to build up pressure to perform conventional plunger lifting. Therefore even if wells are shut in, there are also situations where current plunger lift technology will not work.